An approach to forecasting the potential for flash flood-producing storms is developed, using the notion of basic ingredients. Heavy precipitation is the result of sustained high rainfall rates. In turn, high rainfall rates involve the rapid ascent of air containing substantial water vapor and also depend on the precipitation efficiency. The duration of an event is associated with its speed of movement and the size of the system causing the event along the direction of system movement. This leads naturally to a consideration of the meteorological processes by which these basic ingredients are brought together. A description of those processes and of the types of heavy precipitation-producing storms suggests some of the variety of ways in which heavy precipitation occurs. Since the right mixture of these ingredients can be found in a wide variety of synoptic and mesoscale situations, it is necessary to know which of the ingredients is critical in any given case. By knowing which of the ingredients is most important in any given case, forecasters can concentrate on recognition of the developing heavy precipitation potential as meteorological processes operate. This also helps with the recognition of heavy rain events as they occur, a challenging problem if the potential for such events has not been anticipated. Three brief case examples are presented to illustrate the procedure as it might be applied in operations. The cases are geographically diverse and even illustrate how a nonconvective heavy precipitation event fits within this methodology. The concept of ingredients-based forecasting is discussed as it might apply to a broader spectrum of forecast events than just flash flood forecasting.

A particular class of weather system, the Mesoscale Convective Complex (MCC), is identified, defined, and contrasted with other types of convective weather systems. It is found that MCC systems frequently occur over the central United States, grow to tremendous areal extent, and often persist for periods exceeding 12 h. In addition to widespread beneficial rains, a wide variety of severe convective weather phenomena attends these systems. The development and evolution of MCC systems is not explicitly predicted by operational numerical models even though they are shown to be organized in a distinctly nonrandom mode on scales that cannot be considered subgrid. The MCC is a convectively driven weather system whose physics are not yet understood, much less incorporated into operational parameterization schemes. A preliminary conceptual model of the life cycle of these systems is presented using enhanced, infrared satellite imagery in conjunction with conventional surface and radar data. The outlook for further study and ultimately for the prediction of MCC systems is encouraging since their time and space scalescoupled with their frequent occurrence over the central United States-make them highly amenable to detailed investigation. vective clouds. Operational prediction of convective precipitation has traditionally been perceived as a sub-grid scale problem that can best be handled using statistical techniques in combination with numerical model output [see, for example, papers by Glahn and Lowry (1972); Klein and Glahn (1974); and ^Bermowitz and Zurndorfer (1979)]. However, satellite images during warm season months (March-September) show a high frequency of organized, meso-a scale 2 , convective weather systems over the central United States. It is believed that these systems, which have been named Mesoscale Convective Complexes (MCCs), are a class of convective weather system heretofore unrecognized in the literature. Numerous examples are shown and a definition and hypothesized life cycle (based upon physical characteristics and associated circulations) for MCC weather systems are presented in the following sections.

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Meteorological conditions associated with more than 150 intense convective precipitation events have been examined. These heavy rainfalls caused flash floods and affected most geographic regions of the conterminous United States. Heavy rains associated with weather systems of tropical origin were not considered. Analyses of surface and standard level upperair data were undertaken to identify and define important synoptic and mesoscale mechanisms that act to intensify and focus precipitation events over specific regions. These analyses indicated that three basic meteorological patterns were associated with flash flooding in the central and eastern United States. Heavy convective precipitation episodes that occurred in the West were considered as a separate category event. Climatological characteristics, composite analyses, and upperair data are presented for these four classifications of events.The large variability of associated meteorological patterns and parameters (especially winds aloft) makes identification of necessary conditions for flash flood-producing rainfall quite difficult; however, a number of features were common to many of the events. An advancing middle-level, short-wave trough often helped to trigger and focus thunderstorm activity. The storm areas were often located very near the mid-tropospheric, large-scale ridge position and occurred within normally benign surface pressure patterns. Many of the intense rainfalls occurred during nighttime hours. These elusive characteristics further complicate a difficult forecast problem.